Minor refactor of decode_block for supertx.

Exit early from function when supertx is used, rather than putting
the bulk of the function body in a single conditional.

Change-Id: I41f388a45bd46e4a6ee1c51f26782ed9bddff4e5

vp10/decoder/decodeframe.c

@@ -1280,8 +1280,12 @@ static void decode_block(VP10Decoder *const pbi, MACROBLOCKD *const xd,
   }
 
 #if CONFIG_SUPERTX
-  if (!supertx_enabled) {
-#endif
+  if (supertx_enabled) {
+    xd->corrupted |= vp10_reader_has_error(r);
+    return;
+  }
+#endif  // CONFIG_SUPERTX
+
   if (mbmi->skip) {
     dec_reset_skip_context(xd);
   }
@@ -1420,9 +1424,6 @@ static void decode_block(VP10Decoder *const pbi, MACROBLOCKD *const xd,
         mbmi->has_no_coeffs = 1;  // skip loopfilter
     }
   }
-#if CONFIG_SUPERTX
-  }
-#endif  // CONFIG_SUPERTX
 
   xd->corrupted |= vp10_reader_has_error(r);
 }